Aircraft: EMB ERJ 170/175 ER/LR
Phase of flight: Parked
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural MEL / CDL; Deviation / Discrepancy - Procedural Maintenance; Deviation / Discrepancy - Procedural Published Material / Policy

During maintenance operation on the aircraft; my co-worker and I damaged the thrust reverser. We were scheduled to do overnight maintenance that included RTWO DAY/ QRH INSTALL/ ACMF;FHDB;DVDR;QAR downloads; and lastly a Engine 1 thrust reverser fault. Before plane arrival I did some research as to what needed to be done on the E1 thrust reverser. It was noted that it simply needed an operational test due to previous maintenance. Once the plane arrived me and my co-worker planned we would start with the simple tasks as the RTWO DAY; and begin to start our downloads. Once accomplishing the ACMF/FHDB download. I went back to the shop with a QAR download due to lack of internet;before I stepped away from the plane I had the computer attached to the front of the aircraft where the DVDR download was taking place. This is when I instructed my co-worker to only remove and open the fan cowl for engine 1; since I was focused on the downloads. He mentioned to me he did not have full awareness of the thrust reverser system; which is why I only instructed him to open and release the fan cowling. Once at the shop and completing the QAR download/upload; my lead wanted to tag along since he assigned himself to an airplane that would be arriving near our gate. Upon approach to the plane I noticed that my co worker was unlatching the thrust reverser cowling latches. I mentioned this to my lead since he was in the same vehicle with me and he stepped out to speak with him to notify him. I was not aware of what was spoken between them; because I walked towards the DVDR download that was being done but had failed. I stayed there and retried my download not paying attention as to if the latches were secured on the thrust reverser. Once my download was complete my lead had left to take care of his plane. At this point me and my co-worker were at the flight deck and we started preparing to test the thrust reverser. According to the deferral; the thrust reverser was deactivated and need to be activated. According to the maintenance manual (AMM 78-XX-XX) for reactivation remove screws; fairing plugs; remove lock plates; inhibition bolts from trans cowl. Then install fairing plugs with screws on the transcowl (torqued to spec) Lastly reactivate the ICU clearing the E1 REV INHIBIT. with all of this being accomplished I powered up the APU together and ran hydraulics. At this point I told my coworker to have the reference opened for the operational test of the thrust reverser; which he did. Being a two man job I was on the ground pushing GMO switch to operate the thrust reverser. Being in sync with him on the phone we began to operate the thrust reverser; for a cycle of 3-4 time approximately. During this cycle I heard a very loud cracking sound that didn't seem normal to me due to past experience. I could not see where this sound was coming from during those cycles since I had to have my hand on the GMO switch while operating. At this point I needed a new set of eyes; so I contacted my supervisor explaining to him what I was hearing and also noted to him I had operated the engine 2 thrust reverser to confirm the odd noise coming from ENG1. Being no sound on E2 it confirmed to me something was off. I rechecked the screws I had tightened earlier to see if I misplaced any of them but noticed they were all good. As my supervisor was coming to assist me it never occurred to me to check the thrust reverser cowl latches; since I was not aware if my co-worker had secured them. I had been to occupied and concerned with the failed DVDR which worked for me the second time. When my supervisor arrived we did the same cycles and he heard the same noise. He inspected it and didn't find much until he bent down and noticed one of the latches was slightly opened. We switched places and he held the GMO switch and I laid underneath the engine to see the forward transcowl latch; latched within itself but not securing the cowl together. Once I latched it to secure it; I noticed that the center IFS latch was completely crushed and bent to bits. I also noticed it had damaged around the latch bolts and cracking some of the frames around it. After this; it was inspected by other Mechanics and found significant damage. It was my negligence to double check the work being performed; it should have been a smarter action on my part to double check the work; it was my lack of awareness. Technician reported lack of communication during a thrust reverser operational check resulted in damage to the engine thrust reverser.